Morphologial variation in colonies of Chalara fraxinea isolated from ash (Fraxinus excelsior L.) stems with symptoms of dieback and effects of temperature on colony growth and structure

The present paper presents the results of long-term observations of differences in growth and morphology of colonies of C. fraxinea isolated from ash stems with symptoms of dieback and the results of studies on effects of temperature on growth and morphology of colonies in vitro. Thirty randomly chosen C. fraxinea cultures, originating from six Forest Districts in southern and northern Poland, viz. Andrychów, Gryfice, Kańczuga, Lębork, Miechów and Mircze, were included in the temperature assay. Colony growth and morphology were evaluated in vitro, on malt extract agar. Two replicates of each isolate were incubated at 5, 10, 15, 20, 25 and 30oC in darkness. Colonies of C. fraxinea isolated from necrotic stem tissues of diseased ash trees differed greatly in colour, structure, growth rate, presence of sectors and stromata as well as the intensity of conidial sporulation. Colony characters were often lost in further sub-cultures grown on the same medium. Colonies of C. fraxinea grew at 5-25oC in vitro. Three isolates still grew, though slowly, at 30oC. The optimum temperature for growth was 20oC. Among 30 isolates tested, five grew most quickly at 25oC and four at 15oC. Differences among colony diameters recorded at 5, 10, 15, 20 and 30oC were statistically significant, while differences at 15 and 25oC were not. At some temperatures, statistically significant differences in growth rate were related to the origin of the isolates. There were three main colony colour types. Temperature was the main factor affecting colony morphology in vitro. The formation of sectors was observed in 36% of colonies, pseudoparenchymatous stromata in 10.4% of colonies, and white crystalline substances in 53.3% of colonies. The results of in vitro observations were used for discussion of the effects of temperature on growth and activity of C. fraxinea in vivo.